Methods of forming images by means of diffusion transfer using silver salts such as silver halides etc. are well known. In practical terms, the method involves, for example, processing a photosensitive silver halide emulsion layer which has been subjected to image exposure in an aqueous alkaline bath which contains a developing agent, a silver halide solvent and a film forming agent (thickening agent); reducing the exposed silver halide grains to silver with the developing agent while converting the unexposed silver halide grains to a transferable silver complex salt by means of the silver halide solvent; diffusion transfer of the silver complex salt by inhibition to a silver precipitating agent containing layer (image receiving layer) which is laminated to the aforementioned emulsion layer; and reducing the complex silver salt with a developing agent, with the assistance of the silver precipitating agent to form a silver image.
This method is normally used in film units composed of a photosensitive element consisting of a photosensitive silver halide emulsion layer on a support, an image receiving element consisting of a silver precipitating agent containing image receiving layer on a support, and a processing element containing an active alkaline aqueous solution which contains developing agent, silver halide solvent and film forming agent, inside a rupturable container. The emulsion layer of the photosensitive element is first subjected to image exposure, after which the photosensitive element and the image receiving element are laminated together in such a way that the said emulsion layer is facing the image receiving layer of the image receiving element, while rupturing the processing element and spreading the viscous alkaline aqueous solution between the two layers by passing the unit between a pair of rollers. The film unit is then left to stand for a prescribed length of time and a print with an image formed in the image receiving layer is obtained by peeling the image receiving element away from the photosensitive element.
Methods of forming images by silver salt diffusion transfer using automatic developing machines and involving the use of a developing bath and a fixing bath or an activator bath containing an alkaline reagent and a fixing liquid, or a single development and fixing bath, as used in the printing industry, are also effective.
In silver salt diffusion transfer photographic processes of this type the processing liquid components are involved in both the development and dissolution of the silver halide, and the development of the image on the silver precipitation nuclei, and unwanted side reactions are liable to occur. For example, the silver halide solvent in the processing liquid may act not only to dissolve the silver halide in the photosensitive element but also to inhibit the development of the image on the silver precipitation nuclei. However, if the solvent is included in the photosensitive element the material will not withstand long term storage. Furthermore, toning agents may act not only in the development of the image on the silver precipitation nuclei but also to inhibit the development of the silver halide. Moreover, image stabilizers included in the image receiving layer not only protect the developed silver from oxidation after development but may also inhibit the development of the image on the silver precipitation nuclei and so the image stabilizers must be located in the lower layers of the image receiving element. Consequently, control of timing for image stabilization and reduction of the inhibition of development are very difficult, and in practice conventional units exhibit a considerable risk of development inhibitation. For this reason compounds which release an image stabilizer as a result of the action of an alkaline agent have been used in the past. These methods cannot be used in systems where acetyl cellulose has been alkali saponified in order to render the layer which contains the silver precipitation nuclei hydrophilic.
Moreover, methods in which a fogging agent (nucleating agent) is included in the photosensitive element have been suggested as a means of stopping the development of the photosensitive layer, but these fogging agents are powerful reducing agents and are liable to become deactivated over long periods of time. Furthermore, the difference in reducing activity with respect to the developing agent is subtle; if the compound is much more active than the developing agent then no image will be formed, while if its activity is very weak the compound will be unable to stop development. Hence the release of a highly active fogging agent with good timing after development in undeveloped areas would be ideal but this has not been achieved in practice.